Abstract: Observed global warming has impacted on ecosystems directly and indirectly. Governments, enterprises and common people in the world pay attention to the climate change of mainly global warming. General recognition for coping with the negative impact of climate change is to adopt adaptation and mitigation strategies, which are two aspects of sustainable development supporting each other. According to the definition of "dangerous anthropogenic interference with the climate system" addressed by the Article 2 of UNFCCC, "dangerous climate" to which man can not adapt is taken as the "threshold" of the impact of climate change on different systems. Such a threshold is the integrated result of climate impact level and the adaptive capacity of a system. Lagged studies and uncertainties of climate change make the threshold still unquantified. Therefore, when, where and how would mitigation be implemented has not been clear. This paper focuses on the impact of climate change on ecosystem, and analyzes key issues for determining the threshold, such as the definition, processes, criteria and assessment of ecosystem adaptive capacity. Simulation and assessment of China's ecosystems are performed with the CEVSA model and an artificial neural network (ANN) model. Preliminary results show that ecosystems of China are mainly in ecological baseline, slightly and moderately unadapted states, and the completely unadapted state does not occur. The B2 climate scenario (about 3.2℃ rising) has a certain positive impact on ecosystem in Northeast China, while the A2 climate scenario (about 3.89℃ rising) has a negative impact on ecosystem in East China.

Key words: climate change, adaptation, threshold, ecosystem, China